For example, in a television receiver, in order to correct pincushion distortion, it is known to use a diode modulation circuit in a horizontal deflection circuit.
In the diode modulation circuit, the damper diode of the horizontal deflection circuit is composed of two diodes connected in series, and a pincushion modulation coil is connected in series to a horizontal deflection coil, and the connection middle point of two diodes and the connection middle point of the horizontal deflection coil and pincushion modulation coil are connected through an S-curve correction capacitor. To the diode at the side connected parallel to the pincushion modulation coil, a switching element for correction of pincushion distortion is connected in parallel.
This switching element for correction of pincushion distortion is synchronized with the horizontal pulse, and is turned on in the latter half of the retrace interval of horizontal deflection. The width of this ON period is modulated parabolically in the vertical period. By thus changing the width of the ON period of the switching element for correction of pincushion distortion, the quantity of deflection current distributed from the horizontal deflection coil into the pincushion modulation coil changes periodically, and the deflection current is modulated parabolically in the vertical period, and the pincushion distortion is corrected.
At the same time, since the current flowing in the S-curve correction capacitor is also modulated parabolically in the vertical period, the S-curve correction is also changed in the vertical period, and is smaller in the top and bottom of the screen and large in the center, so that the intermediate pincushion distortion is also corrected.
In the horizontal deflection circuit having such diode modulation circuit, in order to correct the horizontal linearity distortion caused due to presence of internal resistance of the horizontal deflection coil, it is known to connect a horizontal linearity correction coil composed of a magnetically biased saturable reactor in series to the horizontal deflection coil.
When using such diode modulation circuit for correcting pincushion distortion, a sufficiently large correction amount of pincushion distortion is obtained as compared with the conventional pincushion distortion correction circuit using saturable reactor, and therefore it is used in the television receiver or display device using a wide-angle CRT requiring a large correction amount, in particular.
However, a problem occurs when the horizontal deflection circuit using such diode modulation circuit is applied in the television receiver or the like of progressive scanning system, that is, the double speed scanning system coming into use recently. This problem is described below.
In the television receiver of progressive scanning system, since the horizontal deflection frequency is two times as high as that of the ordinary scanning system, that is, interlaced scanning system, the retrace interval of the horizontal deflection current is 1/2. Considering the dielectric strength of the switching element for horizontal output, if the voltage of the retrace pulse is not changed, the maximum amplitude of the horizontal deflection current must be doubled. Accordingly, a switching element of a large capacity is needed, the circuit cost is raised, and the power consumption of the switching element and its peripheral elements is increased.
It is hence desired to employ the horizontal deflection circuit using such diode modulation circuit in the television receiver or the like of double speed scanning system without increasing the power consumption.
Previously, the present applicant proposed, as a horizontal deflection circuit for use in a television receiving using a CRT, a horizontal deflection circuit capable of applying a voltage of about 2 kv to the horizontal deflection coil by using two switching elements, and saving the power consumption and reducing the cost substantially by regulating the horizontal deflection current of the television receiver scanning at double speed to the level of an ordinary television receiver in Japanese Patent Application No. 9-221366 (U.S. patent application Ser. No. 133,992).
In this proposed horizontal deflection circuit, one end of a parallel circuit of a first switching element, a first damper diode, and a first resonance capacitor is grounded, and one end of a parallel circuit of a second switching element, a second damper diode, and a second resonance capacitor, and one end of a primary winding coil of a flyback transformer are connected to other end of the first switching element, a direct-current voltage is supplied to this connection point through the primary winding coil of the flyback transformer, a horizontal deflection coil is connected to other end of the second switching element, an S-curve correction capacitor is connected in series to the horizontal deflection coil, and the other end of the S-curve correction capacitor is grounded, and further switching element control means for controlling the OFF start timing and OFF period of the second switching element is provided.
According to this proposed horizontal deflection circuit, the withstand voltage of the switching element for horizontal output may be low, and the retrace pulse voltage applied to the horizontal deflection coil is large and the deflection current is small, so that the power loss of the deflection system is decreased, and moreover screen size adjustment in the horizontal direction and distortion correction can be done easily.
However, in the horizontal deflection circuit proposed in Japanese Patent Application No. 9-221366, although the S-curve is corrected by the S-curve correction capacitor connected in series to the horizontal deflection coil, the correction amount is not changed, and therefore the intermediate pincushion distortion is not corrected.
To correct the intermediate pincushion distortion, it may be considered to employ a diode modulation system, but since the circuit type is different, intermediate pincushion distortion cannot be corrected by the same method.
Similarly, in the horizontal linearity correction, in the proposed horizontal deflection circuit, since it is different from the horizontal deflection circuit of diode modulation system, horizontal linearity transformer cannot be used, and pincushion imbalance occurs between the right and left side of the screen.
Besides, when the horizontal linearity correction coil or horizontal linearity correction transformer is used, the voltage at both ends applied to the horizontal deflection coil is decreased by the portion of the voltage at both ends applied to the coil. As a result, the capacity and efficiency of the power source cannot be fully utilized, which may lead to problems of increase of power consumption and generation of heat.
Therefore, if the horizontal linearity can be corrected by using a switching element, it is not only advantageous for power consumption, but also easy for control of correction characteristic and possible to lower the cost. It is hence desired to realize a horizontal deflection circuit capable of correcting horizontal linearity by using a switching element.
In the light of the background discussed above, it is an object of the invention to correct intermediate pincushion distortion, horizontal linearity and others as efficiently as in the horizontal deflection circuit of the conventional diode modulation system, in a horizontal deflection circuit using two switching elements, capable of applying a voltage of about 2 kV to the horizontal deflection coil, regulating the horizontal deflection current of scanning at double speed at a level of an ordinary horizontal deflection circuit, saving the power consumption, and lowering the cost substantially.
It is also an object of the invention to keep constant the high voltage direct-current voltage obtained from the secondary winding of the flyback transformer, even in the case of such corrections.